Use of a viral vector is known as a method for delivering a foreign gene into living bodies or cells. Until now various vectors have been developed utilizing a retrovirus, a lentivirus, an adenovirus, an adeno-associated virus, and Sendai virus.
These conventional viral vectors, however, have the following disadvantages. For example, in cases where the vector is a DNA virus, the viral gene is integrated into a host chromosome, thereby exhibiting pathogenicity for the host (for example, humans). Some of the conventional viral vectors have a narrow host range and thus can only be applied to a particular organism. In addition, the gene transfer efficiency of the conventional viral vectors is poor because it varies with the insertion site of a foreign gene into the viral genome. Furthermore, the stability and persistency of the conventional viral vectors are poor due to elimination of the introduced virus by the immune response in a living body, mutation of the viral gene, and/or changes in the promoter efficiency.
In gene therapy and the like, a gene transfer technique capable of introducing a gene into only a target cell has been desired. In particular, since gene therapy is considered to be effective for treating neurological diseases, development of a viral vector capable of selectively introducing a gene into a neuron and having excellent safety, stability, persistency, and gene transfer efficiency has been desired.
Borna disease virus (hereinafter also referred to as BDV) is a neurotropic virus belonging to the order Mononegavirales and having a single nonsegmented minus-strand RNA as a genome. BDV replicates itself in the cell nucleus, and is characterized by non-cytotoxic, prolonged persistent infection and very wide host range (Protein, Nucleic Acid and Enzyme, Vol. 52, No. 10, 1168-1174 (2007); and Keizo Tomonaga, Boruna Byou Uirus Kansen To Tyuusuushinkeikei Shikkan (Borna Disease Virus Infection and Neurological Disorders), Saishin Igaku, Vol. 60, No. 2 (offprint from the February, 2005 issue), 79-85).
A technique, utilizing BDV, for foreign gene transfer into cells has been reported in U. Schneider et al., Journal of Virology, 7293-7296 (2007). In this literature, an expression cassette encoding a green fluorescent protein (GFP) is inserted into the untranslated region at the 5′ end of the BDV genome, and this recombinant virus is used with a high-activity polymerase to infect a rat. As a result, the GFP gene is expressed in the neurons of the rat.
JP-2010-22338-A discloses a viral vector comprising (a) a cDNA encoding a recombinant Borna disease virus genome in which a foreign gene is inserted into the open reading frame of the G gene, (b) a cDNA encoding a ribozyme, and (c) a promoter sequence, each being disposed in a position in which (b) is placed upstream and downstream of (a), and (a) and (b) are placed downstream of (c).
The above technique using BDV would enable selective insertion of a foreign gene into central nervous system cells. However, the gene transfer efficiency of this technique using BDV is not high enough and hence needs to be improved. Thus, so far there has not been developed any viral vector having a high replicative efficiency of a recombinant virus, a wide host range, a high safety, a high foreign gene transfer efficiency, excellent stability and persistency, and a capability of selectively introducing a foreign gene into the central nervous system.